The major objective of this study is to determine to what extent host specificity is affected by host genetics and host distributional patterns. We hypothesize that specificity of parasitism, at least in the coccidia and intestinal helminths which we propose to study in detail, may be determined by these factors and that specific knowledge of pattern and process involved in these relationships should eventually allow us to predict infection patterns in wild populations. The ability to make such predictions would provide valuable data that are necessary to help control parasitic diseases which are potentially harmful to humans and/or livestock and are naturally maintained in wild mammalian reservoirs. To allow valid tests of the predictions of this hypothesis, the extent and nature of genetic variability in a wide array of wild rodent and bat species, collected both in South America and New Mexico, will be determined. The coccidian and intestinal helminth faunas of all host species will be identified and, in selected cases, the genotypes of the parasites also will be determined. These data will then be tested for within and among species correlation of all combinations of variables available. In addition, tests of concordance will be conducted among those species for which gene trees are available to determine if the parasites and hosts have shared a long time association resulting in coevolution of both forms or if the parasites have had a history of host switching. Model systems to be utilized in these experiments will include some groups that are poorly known from tropical South America, some groups that are geologically very old and genetically very conservative, and some that are relatively young and genetically highly variable. Controls will be maintained among those samples and local experiments conducted on the Sevilleta Long Term Ecological Research Site near Albuquerque.